The present invention relates generally to electronic device interconnection technology, and more particularly, to substrates with channels that allow optical interconnections between electronic components.
As integrated circuits (ICs) and other active electronic devices become smaller and more complex, and as the operational frequencies of such devices increase, interconnection technology has become more important. For example, multi-chip modules (MCMs), which integrate several devices into a single package, require sophisticated interconnections between the devices. These interconnections must be reliable, efficient and cost effective to produce. Unfortunately, conventional interconnection technologies provide electrical transmission of signals and have several disadvantages, which are an impediment to the development of higher speed, higher density modules requiring complex interconnects.
In use, ICs are typically mounted on printed circuit boards (PCBsxe2x80x94sometimes referred to as xe2x80x9cprinted wired boardsxe2x80x9d or PWBs) either directly or using interposer substrates, which provide connections to other active and passive devices mounted either on the interposer substrate, on the PCB or which are external to the substrate. Interposers are sometimes used in IC packages to provide high density and increased functionality of the packaged devices. Particularly useful are silicon interposers, fabricated from silicon wafers, which may contain high density interconnects and passive devices. One or more IC xe2x80x9cchipsxe2x80x9d can be mounted on an interposer, which may then, in turn, be mounted on a PCB or other substrate, so that the interposer forms an intermediate packaging level between the ICs and the PCB. A silicon interposer may also contain imbedded passive components, and thereby bring the passive components closer to the IC chips resulting in reduced parasitic interconnect inductances. Furthermore, a silicon interposer allows coefficient of thermal expansion (CTE) matching between the IC and the printed circuit board, and therefore improves the mechanical robustness of the package.
Whether an IC is mounted directly on a PCB, or indirectly using an interposer, it is necessary to provide input and output signal paths to and from the IC. As the density and speed of ICs have increased, the use of electrical signal paths has become a limiting factor. The major causes for delays in conventional inter-chip transmission lines are the capacitive loading of the lines and the line resistivity, which both limit the phase velocity of a propagating electrical signal. In addition, the ability to decrease the spacing between electrical paths is limited by the need to avoid xe2x80x9ccross talkxe2x80x9d between the paths.
Therefore, it would be desirable to have a system that provides complex, high density interconnects that avoids the problems associated with conventional technology, and thereby enables the use of devices having high component densities and high operational frequencies.
In one embodiment of the present invention, a method is provided for transmitting light of at least one wavelength of interest in an substrate having active devices mounted thereon, comprising steps of forming a groove having reflective walls in a generally planar body, mounting at least one additional layer on the body to substantially cover the groove to form an optical transmission channel in the substrate, and providing an optical ports on said substrate for communicating light into and out of the optical transmission channel. One or both of the body and the additional layer may be formed from silicon or a polymer. In one embodiment, the groove is formed by wet etching (001) crystalline silicon such that angled walls are formed as a result of the etching process. In another embodiment the groove is formed in a polymer layer formed on a base layer. In this embodiment it is preferable that the polymer is photo-definable. The walls of the groove may be coated with a material which is reflective to said light, preferably a metal such as aluminum or gold. According to an aspect of the present invention, complex or xe2x80x9c3Dxe2x80x9d light paths may be formed, i.e., light paths within the substrate wherein the light is redirected multiple times. Embodiments used for complex light paths may include multiple layers.
In another aspect the present invention is directed to a substrate for mounting one or more active devices, comprising a body having a groove with reflective walls formed therein, and an additional layer mounted on said body, said additional layer having a surface portion which covers and encloses a major portion of said groove, said surface portion being reflective to said light wavelength, said second body having optical input and output ports formed therein and communicating with said groove, thereby forming an optical path within said substrate.
In another aspect the present inventions comprises a substrate for mounting one or more active devices, having a generally planar rigid base layer, a first polymer layer formed above said rigid base layer, said polymer layer having at least one groove formed therein, said groove having walls that are reflective to a desired wavelength of light, and having at least one angled surface for redirecting light propagating within said groove, a cover layer formed above said first polymer layer, said cover layer having upper and lower surfaces and being substantially transparent to light traveling generally normal to said upper and lower surfaces, and, a light input and a light output for communicating light into and out of said substrate.
In a further aspect, the present invention comprises a method for making a optical transmission path within a substrate, comprising the steps of providing a rigid base layer, forming a first polymer layer over said rigid base layer, said polymer layer having an upper surface, forming at least one groove having at least one angled surface for reflecting light in said polymer layer, coating said groove and said angled surface with a thin film of reflective material, providing a temporary substrate having an upper surface, forming a sacrificial layer on said upper surface of said temporary substrate, forming a second polymer layer on said sacrificial layer, said polymer layer having an upper surface, attaching the upper surface of said first polymer layer to the upper surface of said second polymer layer, and thereafter, removing said sacrificial layer and said temporary substrate.